The emergence of micro fabrication technology has led to a number of developments in the field of airborne acoustic sensors (microphones). Traditionally, the most common technology for microphones has been the use of an electret to detect a deflection of a diaphragm caused by a differential acoustic pressure. Electrets are insulators (such as Teflon or Mylar) on which an electrical charge is trapped. An electret is used in a microphone to produce the necessary electrical field in the air gap between the electrically conductive movable diaphragm and fixed electrode to detect the deflection of the diaphragm. Alternatively, a DC potential between the diaphragm and fixed electrode may be applied from an external source to create the electrical field. This latter device is referred to as a condenser microphone.
A common problem with electret microphones is leakage of electrical charge from the electret, which directly affects the sensitivity of the microphone. This problem is especially severe at elevated temperature and humidity levels. It is inherently difficult to protect the microphone, since it must be exposed to the environment to detect an acoustic signal. Condenser microphones do not suffer from this problem; however due to the large air gaps in devices made with traditional fabrication methods, the external DC potentials required are in the order of hundreds of Volts, which is difficult to realize in battery powered systems.
With the utilization of micro fabrication technology, it is possible to significantly reduce the dimensions, including the air gap, of a microphone. With micro fabrication technology, condenser microphone structures can be fabricated which only require an external DC potential of 5–20 Volts. There are several key motivations for the development of MEMS microphones, the most important of which are: improvement of device ruggedness in system assemblies, miniaturization, improvement of performance and manufacturability of existing devices, and potential monolithic integration with semiconductor electronics.
An important limiting parameter for the performance, specifically sensitivity, of micro fabricated microphones is the mechanical sensitivity of the diaphragm in the device. As the device is scaled down, the microphone sensitivity increases linearly as the air gap decreases, but this is counteracted by a decrease which goes to the fourth power of the diaphragm size. The mechanical sensitivity of the diaphragm is determined by the material properties (such as Young's modulus and Poisson's ratio), thickness, and any intrinsic stress in the diaphragm. It is therefore very important to maximize the diaphragm sensitivity by making it very thin and with a minimal amount of intrinsic stress. In micro fabrication, it is difficult to control the intrinsic stress levels in materials, hence special attention is required to solve this problem. In the prior art, the stress problem has been addressed by using low-stress materials, such as single crystal silicon, polycrystalline silicon and silicon germanium for the diaphragm. Alternatively, the intrinsic stress can be relieved by creating a compliant suspension between the diaphragm and the supporting substrate, which allows the diaphragm to expand and contract.
The idea of suspension is attractive, since it will not only allow relief of any intrinsic stress in the diaphragm, but also decouple the diaphragm from any stress induced due to mismatch of thermal expansion between the diaphragm and the substrate, as well as any stress stemming from the mounting of the substrate in a package. There is, however, some undesirable features associated with prior art devices.
One prior art microphone device 210 shown in FIG. 1 contains a diaphragm 211 supported by four or more springs 212, which are all formed from a silicon substrate 213. However, to realize springs 212 and diaphragm 211, a number of slots 214 must be etched in diaphragm 211, which leads to an acoustical bypass, or leakage, of diaphragm 211. As a result, the low-frequency roll-off of microphone 210 is directly determined by these slots, the dimensions of which are difficult to control. Furthermore, since the motion of diaphragm 211 is set by the stiffness of suspension springs 212, it is important to control tightly the physical dimensions of these springs.
An alternative microphone device 220 shown in FIG. 2 is a variation of the design in which the diaphragm 221 is suspended in a single point 222 (see FIG. 2) or along a straight line around which diaphragm 221 can freely expand or contract. Since the air gap in this type of structure not only defines the distance between movable diaphragm 221 and the fixed counter electrode 223, but also the acoustical leakage resistance in the device it must be tightly controlled. As the diaphragm in this device is essentially a cantilever with one end fixed and the other end free to move, any intrinsic stress gradient in the diaphragm material will cause diaphragm 221 to bend, leading to a change of the air gap in the device, and therefore, the sensitivity and roll-off frequency. This problem is especially important if the diaphragm is composed of more than one material, which may induce a stress gradient by mismatch of thermal expansion in the different materials. Therefore, to realize a suspended diaphragm structure such as that shown in FIG. 2, precise control of dimensions and material stress gradients is required. In another prior art design without suspension in which the diaphragm is loosely confined between the substrate and a lateral restraint, there is no suspension force to release the diaphragm from the substrate, and thus, it is important to avoid stiction in the device during the release of the diaphragm. Unfortunately, surface forces and associated stiction is a predominant effect in micro fabrication due to the extremely smooth surfaces in the device.
Microphones with directional properties are desirable in many applications to lower background noise levels and, in some systems, to enable determination of sound source location. A fundamental limitation on the directivity of a single pressure type microphone is that the size of the sound detecting diaphragm must be comparable to the wavelength of the sound of interest to achieve significant directivity. For human speech and hearing, which is centered around a wavelength of approximately 156 mm, this requires diaphragms of unrealistic sizes. Alternatively, as shown in FIG. 3a, a pressure type microphone 230 can be combined with a pressure gradient microphone in a single structure to achieve a directional response. Such microphones are known as first order gradient microphones. By carefully adjusting the volume of the air cavity 231, the acoustic resistance through the screen 232, and the acoustical path length from the front of the diaphragm 233 to the screen 232, a directivity pattern known as a cardioid pattern can be achieved (see FIG. 3b). The directivity pattern is depicted for a sound source location 235 at the angle θ from a principal direction 234. Microphone 230 has maximum response on the principal axis 234 of microphone 230 and a null response at ±180° from principal axis 234, the principal axis being perpendicular to diaphragm 233. The condition which must be met to achieve the cardioid pattern shown in FIG. 3b is given by:Δl=cCARA,where Δl is the acoustical path between diaphragm 233 and screen 232, c is the speed of sound in air (344 m/s), CA is the acoustical compliance of the air cavity 231, and RA is the acoustical resistance of screen 232. For very small devices, it is difficult and costly to manufacture screen material with high enough acoustical resistance to meet the condition above. Secondly, since the lower roll-off frequency of the microphone is given approximately by:
            f      low        =                  1                  2          ⁢                                          ⁢          π          ⁢                                          ⁢                      R            A                    ⁢                      C            A                              =              c                  2          ⁢                                          ⁢          π          ⁢                                          ⁢          Δ          ⁢                                          ⁢          l                      ,the lower roll-off frequency of microphone 230 increases as the exterior dimensions decrease. As a result, most first order gradient directional microphones have a sloped frequency response, since in most cases the frequency of interest for detection is smaller than the roll-off frequency. Such microphones are typically referred to as having a ski-slope response.
A common method employed to improve the frequency response of a directional microphone is to increase the effective acoustical path Δl by design of the microphone package. FIG. 4 shows a microphone package 240 with two air cavities 241 in which acoustical inlets 242 and 246 for the front and back of the microphone diaphragm 243 are further separated by tubes 247 mounted on the microphone package. The microphone shown in FIG. 3a employs only one damping screen 232; however, if a second damping screen 246 is added in front of diaphragm 243, the frequency response can be leveled when compared to the structure of FIG. 3a with one damping screen. The device of FIG. 4 has symmetric acoustic paths and resistances on both sides of single diaphragm 243.
Another approach to achieve directivity is to implement a so-called second order gradient (SOG) microphone, in which the difference in arrival time of the incoming acoustic wave is enhanced by electronic or acoustical means. The principal idea of the SOG microphone is illustrated in FIG. 5 and FIG. 6. In FIG. 5, the typical electronic implementation of an SOG microphone is shown with an array 250 of four omni directional microphones 251, and a complex summing network 252. A specific time delay τ and τ′ is added electronically to the microphone signals and the signals are subtracted in the network 252. As a result, the output signal 253 of summing network 252 is the sum of the signal from microphone M1 and the signal from microphone M4 delayed by τ+τ′, minus the sum of the signal from microphone M2 delayed by τ′ and the signal from microphone M3 delayed by τ. If τ and τ′ are chosen such that τ=2τ′, the delays will make the microphone array 250 behave as if the distance between each microphone is increased by c*τ′ where c is the speed of sound in air (344 m/s). In other words, a small array can be made to behave like a larger array, with better directivity, by adding delays to each microphone signal. The disadvantages of adding electronic delays are the number of external components needed to realize the functionality, and the need for finely tuned and matched microphones in the array.
It is also possible to achieve the desired delay by acoustical means. Such an implementation is shown in FIG. 6, in which a first order gradient microphone 261 is connected to acoustic paths 262–265 of different lengths which has openings 266–269 with impedance matched acoustic resistances. In operation, the acoustic paths 262–265 act as delay lines, and by adjusting the length of the paths, a directional response similar to the electronic system of FIG. 5 can be realized. A common drawback of all approaches described above is the relative bulkiness of the devices, which does not lend itself well to miniaturization due to fundamental limitations in the underlying physics upon which these devices are based.
An alternative detection principle has been found in nature in auditory organs of the Ormia ochracea parasitoid fly. This insect uses hearing to locate sounds produced by crickets, and has been shown to possess a remarkable directional hearing ability. An impressive feat considering the distance between the eardrums in the insect is only approximately 2% of the wavelength of the sound of interest (4.8 kHz). It has been shown that complex interaction between the two eardrums through mechanical coupling greatly enhances the directional response of each eardrum. A single diaphragm solution with properties similar to the second order mechanical system of the hearing organs in the fly has been suggested in the prior art. The single diaphragm contains a number of corrugations to create resonance modes similar to the dominant vibration modes in the hearing organs of the fly. Unfortunately, the micro fabrication of a single diaphragm with these properties is difficult and problems with stress and stress gradients in the diaphragm material, leading to intrinsic curling and deflection, complicates the matter in a similar fashion as described earlier.